1. Field of the Invention
The present invention relates generally to a thermal energy transfer system for cooling a structure which can easily be added to an existing Freon compression air conditioning system which will replicate the operation of a conventional condensing unit while using only energy which was previously stored in the system and which does not require modification of the components of the air conditioning system normally located within the structure being cooled.
2. Description of the Prior Art
Because the utility electrical industry has incorporated reduced electrical rates in off-peak hours when demand is low, the electrical consumer has found it advantageous to purchase and store air conditioning in the off-peak hours and use it during peak hours. There are many methods of storing and retrieving thermal energy in an insulated tank. All require an insulated tank that contains a substance in which the thermal energy is stored.
One method utilizes a liquid that simply stores the thermal energy by reducing the temperature of the liquid. For example, if this liquid is water, one pound of water stores approximately one BTU per degree of Fahrenheit temperature reduction. The energy is stored by removing heat from the liquid by various methods. The energy is recovered by circulating the cooled liquid into a heat exchanger during peak hours where it absorbs heat because of the low temperature of the liquid.
Another method of thermal energy storage involves the freezing of the liquid inside the insulated tank to its solid state by various methods. The heat stored per pound of liquid is much greater because of the change of state of the liquid to solid. If water is the liquid, one pound of water stores approximately 144 BTU's per degree of Fahrenheit temperature reduction, the phenomenon being referred to as the latent heat. The energy is recovered from storage by circulating a substance (sometimes the same melted liquid) through or around the cold solid transferring heat to the solid until it is all melted back to its liquid state.
Another method of thermal energy storage is a combination of the two previously described methods. Thermal energy is stored by transferring heat out of a liquid until a portion of the liquid solidifies to a solid state resulting in a slurry of solid particles floating in a liquid. Thermal energy is retrieved by circulating the liquid of the slurry to the area to be cooled where heat is added to the cool liquid. The heat is rejected to the particles of solid floating in the slurry.
Because of problems involved in creating the above slurry and thus storing thermal energy, another method has evolved which uses sealed spherical balls containing a liquid that changes to its solid state to store thermal energy. These balls are contained in a liquid that freezes at a much lower temperature than the liquid contained in the balls. Energy is stored by removing heat from the low temperature liquid until the liquid inside the balls changes to the solid state. Energy is recovered by circulating the low temperature liquid to the area to where heat is added and then rejected to the melting of the liquid inside the balls. U.S. Pat. No. 4,768,579, issued to Patry, is an example of this method.
All of these methods have advantages and disadvantages, depending upon the particular end applications, methods of storing and retrieving heat, and commercial considerations of tank size, tank location, etc. All of these methods retrieve the stored energy by circulating a liquid to transfer the heat removed from the air conditioned area to the tank containing the material in which thermal energy is stored.
It has long been recognized that using Freon for the conversion and transfer of thermal energy was beneficial because the conventional method of air conditioning could be used when required during off-peak hours. Past efforts for this method of conversion and storage of thermal energy always used a conventional condensing unit. Past efforts for this method always used a coil submerged in liquid contained in an insulated tank for the thermal energy conversion and storage. These submerged coils had Freon flow through them to freeze the liquid to its solid state for energy storage. The same coil was used for stored energy recovery by flowing Freon through the coil where it condensed to its liquid state, thus adding heat to the frozen liquid in the tank. This method of converting and storing thermal energy is known informally as the “ice on pipe” technique and is described in the American Society of Heating, Refrigeration and Air-Conditioning Engineers Handbook 1998–2001. When using Freon for “ice on pipe” thermal energy conversion and storage, the problem of Freon management becomes increasingly important. Because the coil in the tank is relatively large, it holds large amounts of Freon. The system as a whole has to operate in three different modes:                1. thermal energy storage—making ice;        2. thermal energy retrieval—air conditioning from ice; and        3. conventional air conditioning.        
Each mode requires a different mass of Freon to be in circulation because of the size and use of the coil inside the storage tank. The solution to the problem of Freon management in “ice on pipe” storage systems has been cumbersome with a number of different solutions having been proposed over the years. At least the following issued U.S. Patents deal with this problem: U.S. Pat. No. 4,735,064 Fischer; U.S. Pat. No. 5,211,029 Dean et al; U.S. Pat. No. 4,916,916 Fischer; U.S. Pat. No. 5,255,526 Fischer; U.S. Pat. No. 5,647,225 Fischer; U.S. Pat. No. 5,467,812 Dean et al; U.S. Pat. No. 5,678,626 Dean et al; U.S. Pat. No. 5,682,752 Dean et al.
These solutions are complicated and eliminate the possibility of multiple air conditioning systems using a common storage tank. It is more expensive to provide multiple thermal energy tanks than to provide one tank of the combined volume. These solutions also eliminate any advantage which other thermal energy storage methods might offer. Such systems require the water that is frozen and the coil inside it to be located near the Freon compressor because of pressure losses in the Freon tubing between the compressor and the coil, compressor lubricating oil loss and entrapment in long runs of Freon tubing between the coil and compressor. The additional cost and inconvenience of the copper tubing connecting the coil and the compressor must be taken into consideration when the two are located apart at a relatively great distance. All such systems require one tank for each existing condensing unit and the location of the tank to be relatively close to the condensing unit.
In a typical installation, there are many buildings that are air conditioned by several conventional Freon air conditioners (usually one for each zone inside the building). As a result, a need has arisen for a method for several condensing units to be converted for thermal storage, which method also allows the condensing units to share the same storage tank. Because it has become common practice to mount conventional condensing units on the roof of the structure, a need has arisen for the common energy storage tank to be mounted on the ground and not on the roof due to the prohibitive weight of the storage tank.
Because there are several methods of storing thermal energy in a tank, there is a need for a device that enables a conventional Freon condensing unit to store and then retrieve thermal energy in the tank using any of the above cited methods of thermal energy storage, depending upon the particular situation at hand.